EXTINCTIONS × CLIMATE
The End-Ordovician Extinction
~445 million years ago
Phanerozoic · Paleozoic · Ordovician
The end-Ordovician extinction, about 445 million years ago, is the first of the five great extinction crises and the only one caused by cold, when tropical seas cooled by about five degrees and ice sheets grew.
The end-Ordovician extinction is the first of the five great extinction crises, and the only one whose immediate cause was cold. Tropical seas that had been at 32 to 37 degrees cooled by about five degrees. Ice sheets grew to match and probably exceed those of the last Ice Age, and sea level fell by about a hundred metres. Almost all marine life lived in shelf seas, the shallow seas over the edges of the continents, and these seas drained away. Water without oxygen, charged with hydrogen sulfide, a gas made of hydrogen and sulfur, rose from the depths onto what remained of the shelves. Water without oxygen is called anoxic, and anoxic water charged with hydrogen sulfide is called euxinic. The extinction occurred in two pulses, one at the onset of the glaciation and one at its end.
Why it matters
Species disappeared in very large numbers, but few whole groups, the large branches of the tree of life, were lost, and the structure of the ecosystem persisted. The extinction was therefore massive in taxonomic terms, counted in species lost, and comparatively moderate in ecological terms, that is, in the way living things were organised. Diversity returned to its former level only after at least thirty-five million years.
Dating & uncertainty
The two pulses of extinction fall within the Hirnantian, the last stage of the Ordovician, one near its beginning and one in its middle. Researchers dispute which pulse was the more severe. They also dispute the trigger: some advocate cooling, others ocean chemistry, others volcanic mercury spikes, sudden rises of mercury in the rocks attributed to volcanoes.
Earth history as one day
21:38:512 h 21 min before midnight
if Earth’s 4.54 billion years were compressed into a single day